This invention relates generally to electronic photodiodes, and more particularly relates to avalanche photodiodes.
An electronic avalanche photodiode (APD) is a variation of a semiconducting p-n junction photodiode. An APD is operated-under reverse bias conditions that enable the absorption of an incident photon at the APD to produce secondary charges by impact ionization, lending the term “avalanche.” One avalanche photodiode diode arrangement, known as Geiger-mode avalanche photodiode (GmAPD) operation, employs an avalanche photodiode that is strongly biased beyond the breakdown voltage characteristic of the photodiode to operate in a metastable state. Under this condition, the absorption by the photodiode of even a single photon can cause an avalanche event that gives rise to a detectable electrical current because the avalanche event can produce a voltage signal swing that is sufficient for directly driving CMOS digital logic. GmAPD operation can therefore achieve single-photon detection accuracy with sub-nanosecond time resolution. Imagers based on focal plane arrays (FPAs) of Geiger-mode avalanche photodiodes have demonstrated revolutionary laser radar and optical communication capabilities.
These capabilities are enabled in part by the ability to employ CMOS digital logic in an avalanche photodiode imaging system. For example, a FPA of GmAPDs can be bonded directly to CMOS readout integrated circuits (ROICs) that can be fabricated separately from the FPA. The use of an all-digital readout reduces power, and makes the APD technology more easily scalable to large array sizes than competing technologies employing, e.g., linear-mode APDs or photomultiplier tubes. A large-array GmAPD in which the photodiodes are relatively densely packed can be employed with CMOS ROIC technology to address a wide range of applications, e.g., in terrain mapping, airborne object identification, and communication in high-loss environments such as deep space and under water.
One limitation of a densely packed Geiger-mode APD array is optical cross talk between photodiodes in the array. When operated in or near Geiger-mode, an avalanching photodiode generates many highly energetic electron/hole charge carrier pairs. Some of these charge carriers lose energy by emitting, within the photodiode itself, a spectrum of photons of various wavelengths, some of which can be absorbed and detected at other photodiodes in the photodiode array. Such detection of photons that are secondary, i.e., produced at and coming from a photodiode in the photodiode array rather than from a source external to the array, cause false detection events across the photodiode array. Cross talk is the term used herein to describe this process of false detection across an APD array due to secondary photon emission and absorption.
Cross talk can at a minimum result in blurring of edges in images produced by a photodiode array, or even cause ‘blinding’ of the entire photodiode array so that no image can be produced. Further, as the sites of photodiodes in an array are moved more closely together, the degree and intensity of cross talk increases; low-pitch, high-density photodiode arrays are therefore significantly impacted by cross talk. As a result, as APD array size, array density, and array performance requirements grow, cross talk becomes increasingly limiting factor for achieving the most important applications for APD technology.